Continuous pulping process



July 16, 1963 A. R. SLOMAN CONTINUOUS PULPING PROCESS Filed Jan. 5, 1961 United States Patent 3,097,987 CONTINUOUS PULPING PROCESS Arthur Raymond Sloman, Burnie, Tasmania, Australia, assignor to Associated Pulp and Paper Mills Limited, Melbourne, Victoria, Australia, a corporation of Victoria, Australia Filed Jan. 3, 1961, Ser. No. 80,238 Claims priority, application Australia Feb. 8, 1960 5 Claims. (Cl. 162-17) This invention relates to improvements in the production of paper pulp and other pulps from wood by the continuous process (which term in this specification refers to processes in which the wood charge is moved continuously or intermittently through the digester from a point of entry to a point of discharge), and refers especially to improvements in the cooking of wood by the alkaline processes and neutral and alkaline sulphite processes for the production of paper pulp. The invention is applicable to the soda process, the sulphate (kraft) process, and to those processes employing neutral or alkaline solutions of sulphites, all of which processes are hereinafter referred to by the term alkaline processes. For convenience, the term wood in this specification is used to refer to wood, e.g. in the form of chips at all stages of the process prior to the formation of pulp therefrom by separation of the fibres and their redistribution in a random manner.

In previous digesting processes for isolating cellulose pulps from wood, it has been the practice, in the case of batch and continuous processes, to add the active chemical either at the commencement of the cook or *by injection under pressure during the course of the cook, and, in the case of the continuous process, the chemical and the wood normally move concurrently through the digester.

It is an object of the present invention to provide an improved process for the production of paper pulps and other pulps from wood, whereby greater efficiency can be achieved in carrying out the pulping process, including a reduction in the amount of active chemical required to produce pulps with the same characteristics, the obtainment of increased reaction rates in the cooking process, and a reduction in bleaching requirements for pulp of a given permanganate number. Another object is to produce pulp of lighter colour than is normally achieved by the alkaline processes.

The invention is based essentially on the concept that the preparation of pulps from wood is the result of two functions; one function involving the reactions of cooking chemicals (such as caustic soda and alkali salts such as sodium carbonate, sodium sulphide and sulphite) with the encrusting materials which are those normally extractable with water, caustic soda, benzene and alcohol as well as those described as lignin, in order to render these materials soluble in water; the other function involving diffusion-convection processes which on the one hand provide the mechanism for the entry of cooking chemicals to the woody tissue and on the other hand the mechanism for the removal of the products of chemical reaction from this tissue to the liquor.

We have found that in the cooking of wood for the production of pulp by the alkaline process, it is desirable that a large excess of alkali be present in the late stages of delignification in order that final delignification reactions can proceed at a reasonable rate; and further that low concentration alkali can :be used to carry out those reactions which in a normal cooking system occur when the alkali concentration is at its highest value i.e. at the commencement of the cook.

We have also ascertained that the presence of large amounts of soluble reaction products in the cooking liquor depresses the rate of delignification. Thus in normal cooking the accumulation of soluble reaction products in the liquor as the .cook progresses together with the reduction of alkali content of the liquor result in the rate of delignification being lowered towards the latter stages of the cook. These soluble reaction products have been found to have little or no effect on the rate of reaction of even low concentration alkali in the neutralisation of acid-producing materials normally neutralised at the commencement of a normal cook.

In so-called batch cooking, it is common practice to charge a digester vessel with raw material such as wood chips, and to add to this a solution of cooking chemical and black liquor to provide sufficient liquor to cover the charge of wood. The vessel is then closed, heated to the required cooking temperature and cooking proceeds until the wood is sui'liciently delignified to allow the production of a fibrous pulp with or without mechanical defibration of the wood such as may be obtained by blowing the mass from the \digester and/or defibrating the cooked fibrous material in machines commonly used in the art for this purpose. The charge may be cooled or not and the product removed, or more commonly the charge and the liquor in which it was cooked are blown from the .digester under the pressure of steam into a storage vessel, and the residual cooking chemical and dissolved reaction products are subsequently removed from the fibrous pulp by washing.

In continuous processes as normally practised, wood or other fibrous raw material is introduced into the vessel in such a way as to provide a substantially continuous rate of addition to the cooking vessel, cook ing chemical is added with or to the raw material at or near the point of introduction of the wood to the vessel and the mass is heated to cooking temperature; and the raw material and cooking chemical are passed forward together concurrently through the vessel at substantially the same rate and are ultimately removed substantially continuously as pulp suspended in the cooking liquor. The residual cooking chemical and dissolved reaction products are subsequently removed from the fibrous pulp by washing in equipment designed for this purpose.

We have observed that in normal methods of cooking as previously practised, whether batchwise in conventional digesters or continuously in continuous digesters, solutions containing alkali at high concentration and with relatively low concentrations of wood solids in solution are used at a time when such chemical reactions could be carried out with liquor containing low concentrations of alkali and high concentrations of Wood solids in solution and, further, that as the lignin content of the wood reaches a low value the alkali concentration is at its lowest value (although not at a low value) and wood solids concentration in the liquor at its highest value and both these at a time when the reverse situation would increase the rate of delignification. In normal pulping practice, in order to maintain at an acceptable value the rate of delignification in the latter stages of delignification a quantity of alkali substantially in excess of that required to complete the chemical reactions of pulping must be added at the commencement of the cook and the liquor discharged with the pulp at the conclusion of the cook contains a substantial amount of unneutralised alkali, the concentration of residual alakli being at such a level as to maintain the rate of delignification at the rate required.

We have found that the above-mentioned disadvantages of the existing continuous processes can be substantially overcome by causing the solution or liquor containing the active chemical to pass through the digester vessel in a direction countercurrent to the movement of the wood through the said vessel, so that increasingly digested wood is caused to meet liquor which is increasingly concentrated in active chemical and preferably is decreasingly concentrated in dissolved reaction products.

The present invention, therefore, includes a method for the production of pulp from wood by the alkaline processes in a continuous digester at elevated temperatures and pressure, which comprises feeding wood to the entry point of the digester and causing it to move through the digester to the discharge point, introducing cooking liquor containing the active chemical to the digester at a point where the wood is substantially cooked, causting the cooking liquor to pass through the digester in a direction countercurrent to the movement of the wood through the digester, and discharging the liquor as black liquor from the digester at a point between the point of entry of the cooking liquor and the point of entry of the wood.

In the practical application of this process, a considerable difliculty is met with, since if the liquor containing the active chemical is added to the pulp at or near the point of discharge of the pulp from the digester, a substantial proportion of this liquor would be discharged with the pulp, because of the retention of the liquor in interstices of the fibrous mass. However, we have found it possible to overcome this difiiculty. We have discovered that, in order that substantially all the cooking chemical added to the vessel should flow countercurrent to the wood in the digester, it is necessary to separate or substantially separate the cooking chemical from the substantially cooked wood prior to the discharge of the pulp from the vessel, and the invention includes novel means for achieving this.

According to this invention, the separation of cooking chemical from the substantially cooked wood is effected by subjecting the cooked wood to a countercurrent diffusion-displacement treatment with water after the introduction of the active chemical, that is, in the zone between the point of entry of the cooking chemical and the point of discharge of the pulp from the vessel. The said treatment, which in a continuous vertical digester employing dcwnflowing wood is effected in a diffusiondispl acement zone in the lower portion of the digester, is of sufiicient duration and extent to ensure that substantially all of the cooking chemical has been removed from the wood when it reaches the point of discharge from the digester. Thus at the point of discharge of the pulp from the digester the liquor accompanying the pulp is substantially free of cooking chemical and reaction products. Subsequent washing of the pulp in either diffusers or on rotary pulp washers to remove residual alkali and reaction products in solution may then be unnecessary or may be required to only a limited extent.

An important and novel feature of the invention is that a very high proportion of the active chemical added in the cooking liquor, namely at least 90% and preferably over 98% of such chernioal, is retained in the digester (i.e. is prevented from being discharged with the pulp) and is thus available for cooking the wood.

A further feature of a modification of the invention is the introdur'tion of a prehydrolysis treatment, where for example it is desired to produce rayon-type pulps from hardwoods. Where this is required, water and/ or acid-containing aqueous solution may be introduced to the wood stream in the digester before said wood reaches the point where spent cooking liquor is removed and this acid solution moved countercurrent to the incoming new wood to allow sufficient time at a given temperature for prehydrolysis to be effected. The hydrolyzate is removed substantially continuously at a point at or near the point of introduction of the Wood.

One form of the invention will be described with reference to the apparatus illustrated diagrammatically in the accompanying drawings, wherein FIGURE 1 is a sectional view, partly broken away for convenience of illustration, showing a Kamyr type vertical continuous digester (which has been modified to enable the process of the present invention to be carried out therein) and associated equipment, and FIGURE 2 is a sectional view, also broken away, showing the upper end of the digester further modified to enable a prehydrolysis treatment to be carried out therein.

The digester comprises a cylindrical pressure vessel 10 mounted with its axis vertical to which the raw material is added continuously or intermittently. For convenience we use wood in the form of chips as usually used in the industry.

The wood chips are fed to a hopper 1 1 from which they are transferred through chipmeter 12 and low pressure feeder 12 to a steaming vessel 13 equipped with a screw conveyor 13 by which they are fed through a high pressure feeding device 14 of known design and pipe line 15 to the upper end of the digester 10 at point 16. To assist in the feeding of the chips, liquor is withdrawn from the digester 10 through outlet 45 and pipe line 17 by pump '18 and is delivered to the feeding device 14 from which it is returned to the digester with the chips through pipe line 15. Mechanism 19 of known design comprising a rotating screw 19 within a cylindrical screen 19 is provided in the upper end of the digester for feeding the chips downwardly into the digester and for substantially separating therefrom the liquor which is returned to the feeding device 14 through pipe line 17.

The digester 10 is provided at a point between its upper end and its mid-point with a set of upper internal screens 20 and a discharge outlet 21 for black liquor; and is provided at a point between its mid-point and its lower end with a liquor inlet 22 and with a set of lower internal screens 23 and a liquor outlet 24. The pipe line 25 connected to the outlet 24 connects through a pump 26 with an external steam heater 27 to which steam is supplied through pipe line 28. Pipe line 29 connects the steam heater 27 to the inlet 22. Cooking liquor (white liquor) is supplied through pipe line 30 and pump 31 to the pipe line 29 and thence through the steam heater 27 where it is raised to the required temperature (say C. to C., usually about 180 C.) and is then introduced into the digester 10 through inlet 22. Heating of the incoming cooking liquor may of course be effected by the addition of direct steam to the vessel or other suitable means.

A test cock 32 is provided at a point below the inlet 22.

Pulp is discharged from the lower end of the vessel 10 through the outlet 33 and valve 34, concentrator 35 and blow valve 36. Liquor separated from the discharged pulp by concentrator 35 is returned to the digester 10 at point 37, near its lower end, through pump 38, pipe line 39 and valve 40. Mechanism 41 of known type is provided to facilitate discharge of the pulp from the lower part of the digester.

We also provide an inlet 42 in the lower end of the digester 10 through which water may be introduced into the digester, said water being supplied under pressure through pipe line 43 by means of pump 44.

The section of the digester between cooking liquor inlet 22 and the black liquor outlet 21 is referred to as the cooking zone, and the section of the digester between the water inlet 42 and the cooking liquor inlet 22 is referred to as the diffusion-displacement zone.

Water is caused to flow upwardly in the diffusion-displacement zone countercurrent to the flow of wood and effects removal from the wood of cooking chemical and soluble reaction products contained in the wood passing downwardly through this zone, thereby ensuring:

1) That the pulp discharged from the digester at 33 is substantially free from cooking chemical and soluble reaction products, and

(2) That any cooking chemical which is entrained in the substantially cooked wood and moves therewith into said zone is carried upwardly by the upflowing water into the cooking zone where it is :able to react on uncooked or partially cooked Wood.

The hot cooking liquor introduced at 22, together with the water introduced at 42 and any chemicals dissolved therein during its passage through the diffusion-displacement zone, pass upwardly through the cooking zone countercurrent to the flow of wood through said zone, and black liquor is discharged through outlet 21.

In a practical example of the application of the process of this. invention, a modified Kamyr continuous digester such as that shown in FIGURE 1 of the accompanying drawings and having a capacity of 72 long tons of 0D. pulp per day, was used, the active chemical employed was caustic soda, and water was introduced to the bottom of the digester at inlet 42.

To start the process, the digester was filled with wood chips, cooking liquor and water or black liquor were added in sufficient quantity to cover the chips and to provide sufficient active chemical (in this case 24% NaOH/OD. wood) to complete the cooking of the wood. The mass was heated by circulating the cooking liquor through the external heater 27 until a cooking temperatureof about 176 C. was reached, and the reaction was allowed to proceed while maintaining this temperature until a suitably delignifiedv pulp was produced. The end point of this cock was determined by testing a sample of pulp removed from the .test cock 3 2.

When the wood had been delignified to a normal pulp, water was introduced under pressure by pump 44 through inlet 42 in the base of the digester and was caused to pass upward in the digester, black liquor at the same time being discharged through nozzle 21 and delivered to storage for soda recovery purposes.

As this procedure was carried out, white liquor was introduced continuously into the digester through inlet 22 via pump 31, external steam heater 27, and pipe line 29, liquor being withdrawn through outlet 24 and recirculated through heater 27. This circulation was continued and the temperature in this circulation system was maintained so that the temperature within the digester at the level of the screens 23 from which the liquor was being removed and to which it was being returned was held in the region of the cooking temperature, namely 176 C.

Water at 75 C. was introduced into the digester through inlet 42 and was passed upward-s through the mass for about two hours at an input rate of 50 imperial gallons per minute. (All references to.gallons in this specification are to imperial gallons.) During the cook, the mass in the digester compacted and it was necessary to add wood chips to the top of the digester to fill the vessel before discharge of pulp was conu'nenced. When the digester was filled, discharge of the substantially cooked pulp was commenced through the outlet 33.

As discharge of the pulp was effected, it was neces-' sary to increase the volume of water being admitted to the bottom of the digester through inlet 42 so that the volume of water introduced through said inlet was equal to the sum of the volume of water discharged with the pulp through outlet 33 and the volume of water required to ffow upwards through the mass of material in the diffusion-displacement zone of the vessel. When the production rate of the digester was at the rate 0t 72. long tons of oven dried pulp per 24 hours, the rate of addition of water through nozzle 42 was 150 gallons per minute; the rate of discharge of water with the pulp through outlet 33 was 100 gallons per minute, and the rate .of flow of water upwards in the diffusion-displacement zone of the digester was 50 gallons per minute. The distribution was controlled by the use of the concentrator 35.

White liquor was introduced through inlet 22 in quantity to provide cooking chemical sufficient to cook the wood during its progress from the upper screens 20 near outlet 21 to the lower screens 23 near the inlet 22. At

. 6 the rate of production mentioned, the time of passage of wood between the top of the upper screens 20 and the bottom of the lower screens 23 was of the order of 74 minutes, and the time required for the cooked wood to pass from the lower screens 23 to the bottom of the digester and the outlet 33 was of the order of 100 minutes.

With the continuous addition of wood chips to the top of the digester and removal of the pulp from the bottom, the continuous addition of white liquor to the heating circuit to pipe line 25, heater 27 and the return pipe line 29 and the maintenance of a temperature of about 176 C. at this circuit, the amount of alkali required was of the order of 14% to 15% NaOH on the OD. wood, and with white liquor containing 1.10 lbs. NaOH per gallon the volume added was about 34 gallons per minute. The permanganate number of the pulp produced was between 18 and 22, and required about 4% available chlorine added as =Ca('OCl) to bleach in a single stage treatment to brightness G.E. A pulp cooked to this permanganate number by normal alkaline processes requires about 6% chlorine.

-In this system, the pulp was discharged from the blowvalve 36 of the digester with water containing 0.07 lb. per gallon of total solids and 0021 lb. per gallon total sodium expressed as NaOH, and 0.0012 lb. per gallon of tree caustic soda expressed as NaOH. Thus, of the wood solids made soluble in the cooking process had been removed from the pulp, prior to its discharge, by the action of the water in the diffusion-displacement zone, and of the active alkali added to the system 99% had been prevented from leaving the digester with the pulp and had been employed for cooking the wood by the countercurrent process.

The method employed for feeding wood chips to the Kamyr digester involves the preheating of the wood by direct steam condensation, and in the operations as described above 133 lbs. per minute of steam was condensed. This condensate (which is 13.3 gallons per minute) together with moisture in the wood enter the digester and must form part of the liquor discharged from outlet 21. The moisture in the wood as it enters the preheater was approximately 50% of the wet weight, and with a wood input rate of 250' lbs. OD. wood per minute (equivalent to an output of 72 long tons O.D. pulp per 24 hours) the water entering as moisture in the wood was equivalent to 25 gallons per minute. Hence the Water input with the wood was 38.3 gallons per minute. The water upflow from the bottom of the digester was 50 gallons per minute. (Further it is necessary at times to allow a flow of up to 30-S0 gallons per minute of black liquor with the incoming wood.)

To control the distribution of water added through inlet 42 so that the required upflow of water in the digester is maintained, flow meters (not shown) are provided so that the flow of fibrous material and water or aqueous solution leaving the system through valve 36 can be known; a similar flow meter (not shown) is provided so that the inflow of water through inlet 42 can be shown. The upflow of water in the digester is the flow of water through inlet 42 minus the outflow of water through valve 36. We have discovered that the distribution can be controlled by the use of the concentrator 35 in conjunction with valve 40 and/or by varying the speed of pump 38. There is a strong tendency for water to flow through the outlet 33 rather than to flow upwards through the vessel because of the relative resistance to flow in the two directions. The concentrator 35 is used to withdraw some water from the pulp and this water is returned to the vessel through inlet 37. Contrary to expectations, we have found that this does not result in an increase in the amount of water to be removed by the concentrator 35 but that the stock concentration of the pulp arriving at the concentrator 35 from the digester remains substantially constant and the water returned to the digester at point 37 becomes part of the upflow.

This is an important finding; it would normally be expected that the flow in the bottom concentrator circuit would become an in-circulation, i.e. that the water entering the digester at 37 would dilute the pulp so that the only effect of using the concentrator 35 would be to make it easier to discharge the pulp through outlet 33, but this is not the case.

The volume of upflow water in the digester is controlled so as to ensure a substantial freedom from cooking chemical in the outgoing pulp. A close operational control can be exercised by testing the liquor accompanying pulp which is blown from the test cook 32, and in the example given above the flow was regulated to give a refractometer reading equivalent to about 1.50 lbs. per gallon of total solids.

The liquor balance in the digester is achieved in the following manner:

Assuming that the upflow of water in the digester is U gallons per minute, that the stream condensed in the direct heating of wood in the steaming vessel 13 is determined as equivalent to S gallons of water per minute, that the moisture content and addition rate of wood are fixed so that the water entering the vessel as moisture in the wood is known as M gallons per minute, and that the rate of cooking liquor addition at inlet 22 is W gallons per minute, the rate of outflow of black liquor at outlet 21 is adjusted to be equal to or slightly greater than (U-l-S-l-M-l-W) gallons per minute.

Under these conditions the digester will be very sensitive to minor changes in liquor flows (due for example to changes in moisture in the wood) because if these occur the pressure will vary considerably. To overcome this, and for other reasons, we allow an inflow of black liquor through pump 48 pipe line 48 and inlet 48, the said inflow being controlled by a pressure controller 49 connected to the vessel at 50 and responsive to changes in digester pressure. Thus we maintain an outflow of black liquor at outlet 21 a little greater than that given by the above formula. Such black liquor as is admitted at inlet 48 varies only slightly and the digester pressure is there by maintained substantially constant. This black liquor leaves the digester with other black liquor at outlet 21. This arrangement is also useful in assisting to maintain the temperature at the top of digester below 105 C., which is important in securing regular feeding of the wood chips.

In balancing the liquor flows in the digester when carrying out the process according to the above example, it will be seen that the flow of black liquor from outlet 21 must be maintained at (50+34+38) i.e. 122 gallons per minute, and this liquor contained about 132 lbs. of wood solids and about 37.5 lbs. total sodium (expressed as NaOl-I) i.e. the total dissolved solids were about 170 lbs. Hence the concentration of total solids in the black liquor effluent from outlet 21 was of the order of 1.39 lbs. per gallon which is a satisfactory concentration for delivery to the soda recovery processes.

To maintain effective extraction of the cooking chemical and dissolved reaction products from the cooked wood it is necessary that the rate of upflow of water he not less than the rate at which liquor is being carried downwards in the cooked wood. We have found that with hardwoods the amount of liquor carried downwards in the cooked wood is approximately equal to 0.15 gallon liquor per lb. original oven-dry wood, so that in a digester operating at an output of T lbs. O.D. pulp per minute, and if the yield of unbleached pulp based on original CD. wood is Y%, the amount of liquor moving downward per minute in cooked wood is Y Y gallons per minute 15 X E 37.3 gallons per minute However, in practice we have found that the rate of upflow of water should preferably not be less than 47 gallons per minute under the conditions described above, the time of treatment of the wood in the diffusiondisplacement zone being from 30 to minutes, and consequently in practice a minimum upflow rate of water of T 47 T 15 X 19 gallons per minute is desirable.

In practice an upflow rate of 50 g.p.m. was used successfully.

It will be clear that the most advantageous rate of upflow of water will be the lowest rate that will effectively remove cooking chemical and dissolved reaction products from the cooked wood. increase in upflow rate above this value will dilute the cooking liquor which will result in lower delignification rates in the cooking zone and will also result in higher soda recovery costs due to the concentration of total solids in the black liquor discharged at outlet 21 being undesinably low.

A further application of the process of this invention is the manufacture from suitable woods, e.g. hardwoods, by this process, of pulp suitable for use in the chemical industry, such as the manufacture of viscose for rayon, or the manufacture of pulp for use as a raw material for cellulose acetate production and the like, where a highly purified form of cellulose is required. In this example we employ a further section at the top of the digester !10 as shown in FIGURE 2 of the accompanying drawing, for acid hydrolysis of the wood before it enters the system as described in the earlier example. The digester in FIGURE 2 is provided with an inlet 46 above the outlet 21 and an outlet 47 at a suitable distance above the inlet 46. Water or an aqueous acid solution is introduced at point 46 above the outlet 21 and is heated to the required temperature by an external steam heater (not shown). The water or acid solution moves upwardly countercurrent to the downward flow of the wood chips and is discharged from the vessel at point 47 near the top of the vessel.

According to a further modification (not shown) the flow of liquor and wood may be concurrent during this prehydrolysis.

Modifications of the process as described above can be made in several ways without affecting the principle of the invention. Thus the volume of the different sections of the vessel can if necessary be changed to ensure that, for example, adequate time of treatment is available for the substantial removal of cooking chemical and other substances from the pulp in the displacement-diffusion zone. Also the volume of water admitted near the bottom of the digester can be changed to either vary the eflluent stock concentration of the pulp stream and/ or to vary the rate of upflow of water through the mass.

In the cooking zone temperature and concentration of cooking chemical in the liquor can both be varied from the values given above, in order to control the degree of pulping obtained, so that, for example, if temperature and/or concentration of cooking chemical are increased, the pulp will be of lower lignin content at discharge and vice versa. Alternatively, temperature may be increased and concentration of cooking chemical decreased or vice 9 versa in such a way that the lignin content of the pulp remains substantially constant but the quality of the carbohydrate in the fibrous material in terms of chemical degradation may be varied. The volume of liquor flowing upwards in the cooking zone can also be varied and in combination with control of the temperature of cooking provides another means for controlling pulp quality. In making these changes it is preferred to control the process so that the black liquor effluent from outlet 21 is sub stantially spent of cooking potential, but it will be understood that if higher reaction rates are required these can be achieved by the addition of cooking chemical to provide higher concentrations and this may be carried out to the point Where some excess cooking chemical is discharged in the black liquor leaving the vessel at outlet 21.

A further modification involves the control of the rate of movement of the wood and/ or pulp through the vessel and this rate can be related to the volume of the vessel in its various sections as described and to the cooking and difiusion conditions imposed. Thus the rate of movement may be increased if cooking temperature and/ or chemical concentration are increased.

I claim:

1. A method for the production of pulp from wood by an alkaline process in a generally vertical continuous digester at superatmospheric pressure and elevated temperatures, which comprises feeding black liquor and the wood in the form of chips to an entry point adjacent the top of the digester and causing the chips to move downwardly through the digester to a discharge point adjacent the bottom thereof, the wood moving under gravity through a preliminary treatment zone, a cooking zone and a dilfusion-displacement zone in the digester; the wood chips in the preliminary treatment zone being in contact with black liquor, the black liquor passing through said preliminary treatment zone in a direction co-current to that of the wood; introducing an alkaline cooking liquor, containing active chemical, at an elevated temperature and pressure, into the digester at a point where the wood is substantially cooked; the cooking liquor passing through the cooking zone in a direction countcrcurrent to the movement of wood therethrough, retaining at least 90% of the active chemical contained in the cooking liquor in the digester with the cooked wood and therefore available for use in the cooking of more wood; introducing washing liquid into the digester at a point between the point of entry of the cooking liquor and the point of discharge of the cooked wood from the digester, the washing liquid passing serially through the diffusion-displacement zone and the cooking zone in a direction countcrcurrent to the movement of wood therethrough, the movement of wood through the cooking zone and difiusion-displacement zone being effected without mechanical dis-integration or agitation in the said zones; discharging black liquor from the digester between the preliminary zone and the cooking zone, and discharging cooked wood substantially free of active chemical from the digester at said discharge point.

2. A method for the production of pulp from wood by an alkaline process in a generally vertical continuous digester at superatmospheric pressure and elevated temperatures, which comprises feeding black liquor and the wood in the form of chips to an entry point adjacent the top of the digester and causing the chips to move downwardly through the digester to a discharge point adjacent the bottom thereof, the wood moving under gravity through a preliminary treatment zone, a cooking zone and a diffusion-displacement zone in the digester; the wood chips in the preliminary treatment zone being in contact with black liquor, the black liquor passing through said preliminary treatment zone in a direction co-current to that of the wood; introducing an alkaline cooking liquor, containing active chemical, at an elevated temperature and pressure, into the digester at a point where the wood is substantially cooked; the cooking liquor passing through the cooking zone in a direction countcrcurrent to the movement of wood therethrough, retaining at least of the active chemical contained in the cooking liquor in the digester with the cooked wood and therefore available for use in the cooking of more wood; introducing Water into the digester at a point between the point of entry of the cooking liquor and the point of discharge of the cooked wood from the digester, the water passing serially through the difiusion-displacement zone and the cooking zone in a direction countcrcurrent to the movement of wood therethrough, the movement of the wood through the cooking zone and diffusion-displacement zone being eflected Without mechanical disintegration or agitation in the said zones; discharging black liquor from the digester between the preliminary zone and the cooking zone, and discharging cooked wood substantially free of active chemical from the digester at said discharge point.

3. A method according to claim 2 wherein at least 98% of the active chemical contained in the cooking liquor is prevented from leaving the digester with the cooked wood and therefore available for use in the cooking of more wood.

4. A method according to claim 2 wherein water is extracted from the cooked wood discharged from the digester by means of a concentrator and is returned to the digester, and wherein the upflow of water and liquor in the digester is regulated according to the degree of extraction of water from the discharged wood.

5. A method according to claim 2 wherein the rate of upfiow of water in the digester is not less than where T is the output of the digester in lbs. 0D. pulp per minute and Y is the percentage yield of unbleached pulp based on original CD. wood fed to the digester.

gallons per minute References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD FOR THE PRODUCTION OF PULP FROM WOOD BY AN ALKALINE PROCESS IN A GENERALLY VERTICAL CONTINUOUS DIGESTER AT SUPERATMOSPHERIC PRESSURE AND ELEVATED TEMPERATURES, WHICH COMPRISES FEEDING BLACK LIQUIOR AND THE WOOD IN THE FORM OF CHIPS TO AN ENTRY POINT ADJACENT THE TOP OF THE DIGESTER AND CAUSING THE CHIPS TO MOVE DOWNWARDLY THROUGH THE DIGESTER TO A DISCHARGE POINT ADAJACENT THE BOTTOM THEREOF, THE WOOD MOVING UNDER GRAVITY THROUGH A PRELIMINARY TREATMENT ZONE, A COOKING ZONE AND A DIFFUSION-DISPLACEMENT ZONE IN THE DIGESTER; THE WOOD CHIPS IN THE PRELIMINARY TREATMENT zONE BEING IN CONTACT WITH BLACK LIQUOR, THE BLACK LIQUOR PASSING THROUGH SAID PRELIMINARY TREATMENT ZONE IN A DIRECTION CO-CURRENT TO THAT OF THE WOOD; INTRODUCING AN ALKALINE COOKING LIQUOR, CONTAINING ACTIVE CHEMICAL, AT AN ELEVATED TEMPERATURE AND PRESSURE INTO THE DIGESTER AT A POINT WHERE THE WOOD IS SUBSTANTIALLY COOKED; THE COOKING LIQUOR PASSING THROUGH THE COOKING ZONE IN A DIRECTION COUNTERCURRENT TO THE MOVEMENT OF WOOD THERETHROUGH, RETAINING AT LEAST 90% OF THE ACTIVE CHEMICAL CONTAINED IN THE COOKING LIQUOR IN THE DIGESTER WITH THE COOKED WOOD AND THEREFORE AVAILABLE FOR USE IN THE COOKING OF MORE WOOD; INTRODUCING WASHING LIQUID INTO THE DIGESTER AT A POINT BETWEEN THE POINT OF ENTRY OF THE COOKING LIQUOR AND THE POINT OF DISCHARGE OF THE COOKED WOOD FROM THE DIGESTER, THE WASHING LIQUID PASSING SERIALLY THROUGH THE DIFFUSION-DISPLACEMENT ZONE AND THE COOKING ZONE IN A DIRECTION COUNTERCURRENT TO THE MOVEMENT OF WOOD THERETHOUGH, THE MOVEMENT OF WOOD THROUGH THE COOKING ZONE AND DIFFUSION-DISPLACEMENT ZONE BEING EFFECTED WITHOUT MECHANICAL DISINTEGRATION OR AGITATION IN THE SAID ZONES; DISCHARGING BLACK LIQUOR FROM THE DIGESTER BETWEEN THE PRELIMINARY ZONE AND THE COOKING ZONE, AND DISCHARGING COOKED WOOD SUBSTANTIALY FREE OF ACTIVE CHEMICAL FROM THE DIGESTER AT SAID DISCHARGE POINT 